The genus Coelomomyces is comprised of approximately 60 known species of aquatic parasitic fungi which have been most commonly reported from mosquito larvae. For over three decades there has been considerable interest in developing one or more species of this group as biological control agents because of their documented ability to cause epizootics resulting in high mortalities in larval mosquito populations. Progress on the evaluation of this potential has been slow until recently because the life cycles of these fungi remained unknown and laboratory cultures were not available for experimental studies. Within the past few years it has been determined that several species of Coelomomyces have a complex life cycle involving an obligate alternation of sexual and asexual generations between an intermediate crustacean host, usually a copepod, and a definitive mosquito host, respectively. In this life cycle three motile stages, meiospores, gametes and zygotes are involved in the transmission of the fungus from one host to the other. Now that a generalized life cycle for this group is known, the effect of environmental parameters such as temperature, pH, ionic concentration, light and photoperiod on these motile stages, and hence copepod and mosquito mortality, must be determined. In the present application we propose to use the Anopheles quadrimaculatus/Coelomomyces dodgei/Cyclops vernalis system and a recently developed bioassay to determine the effect of the above variables on the viability and performance of the motile spores. Although a life cycle involving a sexual phase is now apparent for several species of Coelomomyces, an asexual cycle, involving mosquito to mosquito transmission via a diploid zoospore, may also exist. The search for such a cycle will continue through attempts to experimentally produce thin-walled sporangia in pupae. The existing species concept for Coelomomyces has recently come into question as a result of the successful hybridization of C. dodgei and C. punctatus. The validity of the current concept will be examined through characterization of morphological and biological properties of the F1 generation, and F2 if it can be produced.